When an antenna vibrates, it induces phase noise on the signals that it transmits and receives. The physical displacement of an antenna in space caused by vibration induces phase errors on signals transmitted and received by the antenna. An alternate interpretation of this phase error is that acceleration induces time-varying Doppler shifts on the signals. This noise source is significant for microwave and millimeter-wave systems operating in high-vibration environments that require low phase noise at small frequency offsets. Moreover, when the vibrational displacement becomes significant compared to the carrier wavelength, nonlinear effects create phase noise spurs offset from the carrier frequency by integer multiples of the vibration frequencies, and vibration can suppress the power at the carrier frequency.
Some microwave and millimeter-wave wireless technologies operating on mobile platforms require low phase noise at small frequency offsets. Commercial off-the-shelf radio-frequency oscillators are available with g sensitivities down to as low as a few times 10−12 per g, where g is the acceleration of gravity near the earth's surface. At this level, the phase noise contribution from the vibration of the antenna will be larger than the phase noise contribution from the vibration of the oscillator at frequency offsets up to several kilohertz. For these systems, antenna vibration dominates the system phase noise. This vibration-induced phase noise degrades communications links, degrades clutter rejection for radars, interferes with radar Doppler velocity measurement of slow-moving objects, degrades synthetic aperture radar (SAR), and degrades navigation accuracy.